Conventionally, there has been known a permanent magnet motor including a shaft, a rotor provided with a rotor core fixed to the shaft and a plurality of permanent magnets serving as magnetic field means arranged at equal intervals in a ring shape on the outside peripheral surface of the rotor core, and a stator having a coil wound on a stator core.
It has been known that cogging torque (a change in torque, which is produced by a magnetic attraction force generated between the stator and the rotor, with respect to the rotation angle) and torque ripple (torque pulsation), both commonly found in the permanent magnet motor, depend on the air-gap magnetic flux density distribution generated between the permanent magnet constituting the rotor and the stator core constituting the stator. The cogging torque and the torque ripple are problematic because they may cause vibrations and a noise in the motor.
As means for solving those problems, there has been known a permanent magnet motor that reduces cogging torque by means of a permanent magnet 5 shown in FIG. 5 (see Patent Document 1: Japanese Patent Application Publication No. 2002-84695). FIG. 5 is a cross-sectional view of the permanent magnet 5 used for the permanent magnet motor described in Patent Document 1. As shown in FIG. 5, for the permanent magnet 5, an inside periphery 51 thereof is formed by a first arc 511, and an outside periphery 52 thereof is formed by a second arc 521, a third arc 522 joining to one end C of the second arc 521, and a fourth arc 523 joining to the other end C′ of the second arc 521.
In the case of a permanent magnet motor (not shown) using this permanent magnet 5, since the second arc 521 is formed by an arc having a center O1 that is the same as the center of the first arc 511, the thickness in the radial direction of the permanent magnet 5 can be made fixed in a zone Z1 (hatched portion) defined by the first arc 511 and the second arc 521, so that the decrease in motor output can be suppressed. On the other hand, in a zone Z2 defined by the first arc 511 and the third arc 522 and a zone Z3 defined by the first arc 511 and the fourth arc 523, the thickness in the radial direction decreases toward the end part, so that cogging torque can be reduced to some degree.
However, in the permanent magnet 5, although the thickness in the radial direction of the permanent magnet 5 is fixed in the zone 1, the thickness in the radial direction in the zones Z2 and Z3 decreases abruptly toward opposite ends B, B′ on the outside periphery 52, so that cogging torque and torque ripple cannot be reduced sufficiently.